1. Field of the Invention
The present invention relates to a magnetic disk apparatus for writing and reading data to and from a magnetic recording medium, and to a semiconductor integrated circuit device for use in such an apparatus.
2. Description of the Prior Art
An IC (integrated circuit) designed for use in an FDD (floppy disk drive) (hereafter such an IC will be referred to as an "FDD IC") incorporates analog circuits that handle analog signals, e.g. a read/write circuit for writing and reading data to and from a magnetic disk, as well as digital circuits that handle digital signals, e.g. various control circuits. In most cases, all of these circuits operate on a common supply voltage (typically 5 V). Operating all the circuits on a single supply voltage, however, causes the supply voltage to be contaminated, and thereby made to fluctuate, by the noise resulting from the operation of the digital circuits. This adversely affects the characteristics of the analog circuits. For this reason, in a conventional FDD IC, the circuits incorporated therein are individually provided with a supply voltage terminal to be connected to the supply voltage and a ground terminal to be grounded.
However, in an FDD IC, even if the circuits incorporated therein are individually provided with a supply voltage terminal and a ground terminal, they invariably need to be connected externally to a single supply voltage. This makes it difficult to sufficiently suppress the degradation of the characteristics of the analog circuits, in particular in cases where the analog circuits handle weak signals. Degradation of the characteristics of the analog circuits, more specifically degradation of the characteristics of the read/write circuit, means nothing but degradation of the accuracy with which the FDD apparatus as a whole writes and reads data to and from a magnetic disk.
Moreover, an FDD IC suffers from large current consumption exactly because all of the circuits incorporated therein operate on a common supply voltage.
Moreover, the manufacturing process of an FDD IC includes a high-temperature test that is conducted to check whether the IC operates normally under high temperature and thereby ensure its reliability. This high temperature test requires not only a high-temperature environment, and thus large-scale testing facilities and equipment, but also much time. Accordingly, an FDD IC tends to demand high testing costs, making the costs of the FDD IC as a whole, and thus the costs of the FDD apparatus employing it, unnecessarily high.
Moreover, it is inevitable that the MOS (metal-oxide semiconductor) transistors forming the digital circuits show a certain degree of variation in their performance. If a circuit happens to be composed of MOS transistors of relatively poor performance, the circuit will not offer the desired characteristics. Since there is no way to save such a circuit, it is not possible to achieve a satisfactory yield of an FDD IC. This makes the costs of the FDD IC, and thus the costs of the FDD apparatus employing it, even higher.